Based on the electromagnetic model for four-port resonant filter, the optical characteristics of several filters with rectangular cascade resonators have been numerically simulated by utilizing two-dimensional FDTD method. The relationship between the number of cascade resonators and the filtering features is analyzed. The narrow-band filter with four cascade resonators implemented on GaAlAs/GaAs planar waveguide is designed and fabricated. The experiment result shows the half-width of filtering spectrum for the chip is about 10 nm, which coincides well with that of the simulation.